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Full text of "The manufacture and properties of iron and steel"

CHAPTER III.
WROUGHT-IRON.
SECTION Ilia.—The puddling process.—When pig-iron is melted on. a hearth of iron ore and is exposed to the action of the flame, there is a rapid oxidation of the metalloids. The silicon, manganese, sulphur and phosphorus unite with oxygen to form a slag, while the carbon escapes as carbonic oxide and carbonic acid. The iron then becomes less fusible, and in an ordinary reverberatory furnace the heat is not sufficient to keep the mass liquid. It becomes viscous, then pasty, and finally is worked into balls, taken from the furnace, and squeezed or hammered into a bloom.
The crude puddle-ball is made up of an innumerable number of globules.of nearly pure iron, while the interstices between the particles are filled with slag. The squeezer expels much of this slag and each subsequent rolling removes a further quantity, but it is impossible to get rid of all the cinder, and it forms a skeleton which permeates the finished bar, forming planes of separation between the particles of metallic iron. These films weaken the material by destroying the cohesion of the particles, but in other ways they are of benefit, for the sulphur and phosphorus are never entirely removed in puddling, and there is usually a sufficient percentage of them left to give bad results if they were able to exert their full effect in producing crystallization, but the network of slag prevents the tendency to crystallize. If bar-iron be melted in a crucible, the slag separates and the impurities have a chance to exert their full force. Some pure irons will successfully undergo this test, but most brands give a worthless metal after fusion. The first rolling of the puddle-ball gives acrude product known as muck-bar. For" the making of merchant iron, this intermediate product, together with miscellaneous wrought-iron scrap, is bundled into "piles" and rolled into the desired shape.
SEO. Illb.—Effect of silicon, manganese and carbon.—The char-